The beaver (Castor canadensis) impacts the natural environment of North America more than any other vertebrate species, besides man. Joel Snodgrass surveyed beaver-created patches on the Savannah River Site during the 1990′s and documented 9 different kinds of habitats. He began his study by examining aerial photographs of the SRS. Beavers had been extirpated from the region by 1950, the year they were first protected by law. Photos from 1950 showed no beaver activity or presence at all. But beavers recolonized the site, and aerial photos from 1978 showed 24 active beaver ponds and 27 patches of land influenced by beavers. Beaver populations continued to rise and photos from 1992 showed 37 active dams and 49 patches of beaver-created habitat. Beavers so successfully recolonized Georgia and South Carolina that they were removed from protected status in 1983, and their populations are now controlled to prevent the flooding of roads, residences, and valuable timber.
The following is a list of beaver-created habitat recognized by Joel Snodgrass in his study of the SRS.
1. Flooded hardwoods–When beavers first dam a stream the impounded water floods such bottomland species as tulip, sweetgum, red maple, and tupelo.
2. Open water–Eventually the trees die and sunlight feeds aquatic plants such as pondweed, water milfoil, and bladderwort (a carnivorous plant). Smartweed and arrow arum (duck potato) attract ducks. Charles Wharton studied a 3 pond beaver complex in Fayette County near the Flint River and recorded a variety of fish including bowfin, pickerel, creek chubsucker, mosquito fish, shiners, small mouth buffalo, crappie, largemouth bass, bream, and darters. He also found frogs, salamanders, snakes, 5 species of turtles, King and Virginia rails, bitterns, green herons, red-winged blackbirds, warblers, ducks, muskrats, raccons, rice rats, and swamp rabbits. A hunter told him he’d seen at least 1000 ducks on the pond during hunting season.
Beaver pond and standing dead wood at Cobb Creek, Toombs County. Beaver ponds are habitat for many species of aquatic wildlife. Photo by Alan Cressler.
Aerial photo of 3 pond complex created by beavers near the Flint River from Charles Wharton’s book The Natural Environments of Georgia. He found these rich in aquatic wildlife.
3. Emergent marsh–Sediment gradually builds in the pond and rushes (Juncus sp.) and bulrushes establish a foothold.
4. Wet shrub–More sediment leads to a growth of shrubs such as buttonbush, alder, and Virginia sweetspire. Buttonbush and alder formed an important part of the mastodon’s diet in the south when that extinct species used to live in the region. A study of fossil mastodon dung found in the Aucilla River, Florida found buttonbush (Cephalanthus occidentalis) to be the leading item in its diet along with cypress twigs.
Buttonbush. A favorite food of the mastodon. It’s a dominant plant in some stages of beaver pond succession.
5. Wet meadow–Eventually sediment completely fills the pond and beavers remove most of the trees in the vicinity, allowing panic grasses, bluestem grasses, and sedges to dominate. During the Pleistocene mammoths, bison, and horses would have been attracted to beaver-created meadows.
Beaver pond. Meadow stage.
Beaver pond. Shrub stage.
6. Dry meadow–The wet meadow dries and bluestem grasses, daiseys, and blackberry brambles take over.
7. Dry shrub–Buttonbush, wax myrtle, alder, and Virginia sweetspire dominate this stage of succession.
8. Dry dead hardwoods–Stands of dead tulip, red maple, sweetgum, and tupelo occur when beavers flood and kill the trees and something (a predator or human trapper) removes the beaver after just a short period of maintenance.
9. Early forest succession–Dry meadows and dry shrubs eventually give way to pioneer red maple, tulip, alder, and Virginia sweetspire.
On the SRS 2/3rds of the streams beavers dam are considered second order streams. Geologists rank streams from smallest (first) to largest (twelfth). 2nd order streams are located near the headwaters and usually have just 1 tributary. They flow into larger streams. Beavers do sometimes dam larger streams, but most simply dig tunnel dens in the sides of larger bodies of water, such as rivers.
A >40,000 year old fossil beaver dam was found in a kaolin clay mine in Deepstep, Georgia. Most of the cut wood was from cypress trees. Castor canadensis co-existed for 2 million years with a much larger species of beaver–Casteroides ohioensis. Fossil evidence of both species has been unearthed in Tennessee, Alabama, Georgia, South Carolina, and Florida. The giant beaver grew to more than 300 pounds. It was not ancestral to the modern beaver. Scientists don’t know whether or not the giant beaver also built dams. The evidence against is based on dental morphology. The giant beaver mostly ate succulent aquatic vegetation, and its teeth were not as well suited to gnawing wood as the modern beaver. However, Natalia Rybczinski, a Canadian paleontologist, studied the teeth of Dipoides, an extinct Pliocene-aged beaver found associated with a fossil beaver dam, and she told me its teeth were even less suited for gnawing wood than those of Casteroides ohioensis; yet fossils of this species were found associated with a beaver dam and lodge with marks from cut wood matching its teeth. She thinks dam and lodge-building are activities so instinctive in the beaver family that all extinct aquatic species of beaver must have built dams and lodges to some degree, though the modern extant beavers have more efficient teeth for this purpose.
Almost every paper on Casteroides includes the phrase, “there’s no evidence giant beavers built dams like their modern cousins.” I recently discovered this isn’t true. A fossil skull of Casteroides was discovered in a fossil beaver lodge found in New Knoxville, Ohio, and the find was documented in a Geology Bulletin published in 1905. It’s possible the giant beaver was using a lodge built by its smaller cousin. (Whoever discovered this site didn’t think to compare the marks on the cut wood with giant beaver teeth to see if they matched. The skull is housed at the Cleveland Museum. The fossil wood’s probably been lost.) Nevertheless, I think that it’s pretty good evidence giant beavers built dams and lodges like their modern cousins. I think giant beavers did build dams and lodges but weren’t as dependent upon them for survival as their smaller cousins and could survive in wetlands that were sparsely wooded because their greater size allowed them to fight off predators with more success.
Modern beaver colonies abandon their lodges after a decade because they deplete their food supply. They can’t risk long overland journeys for wood because it puts them in danger of being killed by predators. They may return to abandoned ponds after enough woody shrubs grow back. During the Pleistocene, megafauna grazing and browsing probably delayed the return of the woody shrub stage, keeping the patch as a perpetual grassy marsh likely favored by the giant beaver. I think humans overhunted giant beavers to extinction, but it could be that the extinction of mastodons and mammoths caused a collapse in the ecological interplay between Castor canadensis and the megafauna in creating the kind of habitat giant beavers required.
Illustration of giant beaver and size comparison between Casteroides ohioensis and Castor canadensis. I think the greater size of the giant beaver lessened its dependence upon trees and dam-making, though it did build dams on occasion.
“Temporal and Spatial Dynamics of Beaver-Created Patches as Influenced by Management Practices in Southeastern North America”
Journal of Applied Ecology 34 1997
The Natural Environments of Georgia
Georgia Department of Natural Resources 1978
“History of Ohio and Anglaize County”
Geological Survey of Ohio 16 19o5 (cited erroneously as 1912 in another paper)